You first have to create an user-role with the name ‘requestor’. Do this in your Mendix modeler when security settings are not “Off”. Add the user-role at the Project->Security->Tab User roles
The properties of the “retrieve UserRole from database” are visible in the image, in the annotation above that activity.
See this forum post about how to retrieve a specific userrole:
https://forum.mendix.com/link/questions/89358
NOTE; the annotation shows a wrong XPath; Never use a hardcoded string in XPath.
In this case use the available token instead